Sea Salt Aerosols in Global Climate Models: Emissions, Transport, and Geoengineering Implications

Abstract

Sea salt aerosols (SSA) play a vital role in Earth’s climate system by influencing radiative balance and cloud formation. However, discrepancies between observed and modeled SSA concentrations create uncertainties in climate projections. This dissertation examines SSA emissions, radiative effects, and interactions with the global climate, focusing on their role in atmospheric processes, climate modeling, and geoengineering. Chapter 1 identifies emission errors, vertical transport errors, and subgrid variability as key sources of discrepancies between modeled and observed SSA concentrations. By combining in situ measurements from NOAA PMEL cruise expeditions with GEOS-Chem model simulations, this chapter highlights the need for integrated approaches to improve SSA representation in global climate models (GCMs). Chapter 2 evaluates the performance of various sea spray generation functions in representing SSA emissions and aerosol optical depth (AOD). Comparisons of four sea spray generation functions in a NOAA GFDL atmosphere model reveal that simulations using the mechanistic formulation by \citet{deike2022mechanistic} achieve the highest correlation with global AOD observations. This chapter emphasizes the regional variability in SSA emission simulations and the challenges of reconciling AOD retrieval methodologies with model outputs. Chapter 3 explores the climatic impacts of geoengineering scenarios by examining the rapid adjustments and atmospheric circulation responses to increased tropical SSA emissions under the G4sea-salt protocol in a NOAA GFDL coupled model. Enhanced tropical SSA emissions induce cooling, reducing equator-to-midlatitude temperature gradients and weakening surface westerlies, which in turn partially offset increased emissions. The analysis reveals significant differences in climate responses to zonally uniform versus regionally targeted SSA geoengineering scenarios, providing critical insights for marine cloud brightening strategies. Chapter 4 assesses radiative forcing and climate feedback parameters in the context of SSA geoengineering, comparing G4sea-salt simulations with the SSP2-4.5 baseline. By isolating the effects of SSA geoengineering, this chapter demonstrates that geoengineered forcings and feedbacks can be analyzed similarly to traditional climate forcings, though nonlinearities in Earth's energy imbalance (EEI) complicate attribution. Collectively, this dissertation advances the understanding of SSA dynamics, their representation in climate models, and their potential role in geoengineering, providing critical insights for future climate mitigation and adaptation strategies.